hi guys,
Went to job today 2 year old property. Combination valve under kitchen cupboard. Connected to the water main stop cock double check valve then combination valve. It has been activating And dripping outside? Upstairs unvented cylinder and tested it and is charged. And no drips at the tundish near the cylinder.

So I think they have installed it instead of a proper pressure reducing valve without a blow off. I'll fit a proper pressure reducing valve.
Do you think it's been incorrectly installed cause they haven't had the right material on site.

It has to be installed at the unvented cylinder.
Some reasons for that probably are,
It is a serviceable part of the unvented cylinder and has couple of things for safety - as an incoming mains pressure limiter and the built in (normally), pressure relief valve.
Needs to have discharge pipe at cylinder from that combination valve so it goes through tundish for visual inspection.
Filter in valve has to be cleaned on some services.

The incoming main can have another pressure reducing valve on it to provide total balance to hot and cold outlets, if the balanced cold from combination valve is not used, often due to existing gravity systems being converted to unvented pressurised systems.

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